Process for Producing Propylene and Aromatics from Butenes by Metathesis and Aromatization

ABSTRACT

The invention is for a process for producing propylene and hexene (along with ethylene, pentenes, product butenes, heptenes and octenes) by metathesis from butenes (iso-, 1- and cis and trans 2-) and pentenes and then aromatizing the hexenes (along with higher olefins, such as heptenes and octenes) to benzene (along with toluene, xylenes, ethylbenzene and styrene). Since the desired products of the metathesis reaction are propylene and hexene, the feed to the metathesis reaction has a molar ratio for 1-butene:2-butene which favors production of propylene and 3-hexene with the concentration of hexenes and higher olefins in the metathesis product being up to 30 mole %. An isomerization reactor may be used to obtain the desired molar ratio of 1-butene:2-butene for the feed composition into the metathesis reactor. After the metathesis reaction, of hexene and higher olefins are separated for aromatization to benzene and other aromatics.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for producing propylene, hexenesand, optionally, ethylene, by metathesis from butenes and pentenes andthen aromatizing the hexenes to benzene.

2. Description of the Prior Art

Olefin metathesis is the catalytic double disproportionation by cleavageand reformation of the carbon double bond (C═C) of two olefin compoundsto form two different olefin compounds. This reaction is reversible, andthe overall product distribution is limited by a thermodynamicequilibrium. The catalysts are generally homogeneous or heterogeneoustransition metal compounds, particularly compounds of transition metalGroups VI to VIII.

U.S. Pat. No. 6,646,172 discloses a multistage process for preparingpropylene from metathesis of butenes and from separate metathesis of theC₅ alkenes produced in the metathesis of butenes.

U.S. Pat. No. 6,580,009 discloses a process for preparing propylene andhexene by metathesis of a raffinate II feedstream of olefinic C₄hydrocarbons and ethene with fractional distillation to recover butanes,butenes and a pentene-containing fraction for recycle. This patent alsodiscloses methods for the removal of butadiene, acetylenic compounds,and oxygen-containing impurities from a C₄ feedstream.

U.S. Pat. No. 6,538,168 discloses a process for preparing C₅-C₆ olefinsby metathesizing butenes into a mixture of C₂-C₆ olefins and butanes,separating C₂-C₃ olefins from C₄-C₆ olefins, further separating C₄-C₆olefins into hexene and methylpentene and a recycle of butanes, butenes,pentene and methylbutene.

U.S. Pat. No. 6,777,582 discloses a process for metathesis of 1-buteneand 2-butene to propylene and 2-pentene which is recycled with productbutenes to foam propylene and hexene.

Aromatization of alkenes is a process of dehydrogenation, cyclizationand aromatization of the alkene. The catalyst for this process must bemulti-functional to have an acceptable conversion and selectivity forthe desired products. Aluminosilicate zeolites are known catalysts foraromatization. Some zeolite catalysts for aromatization contain a GroupVIII deposited metal, such as platinum, and elements other than siliconand aluminum, such as germanium, in the zeolite crystalline framework.

U.S. Patent Application Publication no. 2008/0255398 discloses a processfor the aromatization of C₆ to C₁₂ alkanes, such as hexane, heptane andoctane, to aromatics, such as benzene, ethyl benzene, toluene andxylenes, with a non-acidic aluminum-silicon-germanium MFI zeolite onwhich platinum has been deposited. One catalyst for the aromatization ofC₆ to C₁₂ alkanes is Pt/CsGeZSM-5.

Metathesis of butenes is known in the prior art. It is known thatmetathesis of 1-butene, 2-butene and isobutene will yield propylene, adesirable product which can be further processed into other compounds,such as acrylic acid, acrylonitrile, isopropanol, propylene oxide, andinto other products, such as polypropylene. Commercialization ofmetathesis of butenes has been impeded due to the lack of uses for theother products of the metathesis reaction, such as hexene and higherolefins. Since metathesis is an equilibrium reaction, these productswhich have been less desirable than propylene are produced in an amountbeyond what is useful for their available market. It would beadvantageous to convert these products to more marketable products.

SUMMARY OF THE INVENTION

The present invention is for a multi-step process which producespropylene and hexene (along with ethylene, pentenes, product butenes,heptenes and octenes) from butenes (1-, cis and trans 2- and iso-) bymetathesis followed by aromatization of the hexene (along with higherolefins, such as heptenes and octenes) to benzene (along with toluene,xylenes, ethylbenzene and styrene). Pentenes and butenes from themetathesis step are recycled. Ethylene may also be recycled or separatedfor further processing. Propylene is separated for further processing.Any metathesis catalyst could be used for metathesizing butenes. Anyaromatization catalyst could be used for aromatizing hexene and higherolefins.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily understood by reference to thefollowing detailed description when considered in connection with theaccompanying drawings:

FIG. 1 is a diagram of a multistage process for metathesis of butenes topropylene and hexene and aromatization of hexene to benzene

FIG. 2 is a graph comparing the calculated equilibrium and observedproduct distributions for metathesis of butenes with a rhenium catalystat 48° C. and 25% 1-butene in feed (Example 1)

FIG. 3 is a graph comparing the calculated equilibrium and observedproduct distributions for metathesis of butenes with a rhenium catalystat 48° C. and 75% 1-butene in feed (Example 2)

FIG. 4 is a graph comparing the calculated equilibrium and observedproduct distributions for metathesis of butenes with a cobalt/molybdenumcatalyst at 97° C. and 50% 1-butene in feed (Example 3)

FIG. 5 is a graph of the observed product distribution for metathesis ofbutenes with a 6% tungsten oxide catalyst at 296° C. and 50% 1-butene infeed (Example 4)

DETAILED DESCRIPTION OF THE INVENTION

The process of the present invention produces propylene and aromaticsfrom feedstreams containing butenes, especially those containing1-butene and 2-butene. The feedstreams may also contain pentenes. Oneexample of feedstock is what is known as “raffinate II” obtained fromthe processing of crude C₄ cuts by removing butadiene from the crude C₄cuts in a first step (“raffinate 1”) and removing isobutene from the C₄stream in a second step. Raffinate II consists of mainly 1-butene and amixture of cis and trans 2-butene. Other examples of feedstreams includecracker byproduct streams, which would contain a mixture of C₄ andpossibly C₅ olefins; and streams obtained from dehydrogenation of butaneand isobutane, which would contain 1-butene, 2-butene, and isobutene.

The present invention is for a process for producing propylene andaromatics comprising:

a) reacting a hydrocarbon feed mixture comprising 1-butene, 2-buteneand, optionally, isobutene in the presence of a metathesis catalyst atconditions to produce ethylene, propylene, product butenes, pentenes,hexenes and higher olefins;

b1) separating and recycling the pentenes, butenes, and optionally, theethylene from the product;

b2) separating propylene from the product;

b3) separating hexenes and higher olefins from the product; and

c) reacting hexenes and higher olefins in the presence of anaromatization catalyst.

A metathesis catalyst can be used to convert a feed containing butenes,and possibly pentenes, into a mixture of ethylene, propylene, butenes(product and unreacted), pentenes (product and unreacted), hexenes andhigher olefins. The pentenes, the butenes, and possibly the ethylene areseparated and recycled to the reactor for further conversion. Thepropylene is separated for further processing. The hexene and higherolefins are separated for aromatization.

Any metathesis catalysts could be used in the present invention. Knownmetathesis catalysts include supported transition metal compounds oftungsten, molybdenum or rhenium, such as WO₃ on SiO₂, MoO₃ on Al₂O₃, orRe₂O₇ on Al₂O₃, and molecular catalysts such as Grubbs' or Schrock'scatalysts. Metathesis catalysts may include promoters, such as compoundsof transition metals, e.g., cobalt, or of alkali metals, e.g., potassiumor cesium. One example of a metathesis catalyst with a promoter isCoO/MoO₃ on Al₂O₃. If butenes and pentenes are present in the feedstreamand/or recycle, there are several possible metathesis reactions. Somepossibilities are given below:

1-butene+2-butene

2-pentene+propylene

2-pentene+1-butene

3-hexene+propylene

2-pentene+2-pentene

3-hexene+2-butene

1-butene+1-butene

3-hexene+ethylene

1-butene+propylene

2-pentene+ethylene

2-butene+ethylene

propylene+propylene

isobutene+isobutene

ethylene+2,3-dimethyl 2-butene

isobutene+1-butene

ethylene+2-methyl 2-pentene

isobutene+2-butene

propylene+2-methyl 2-butene

1-pentene+1-butene

3-heptene+ethylene

1-pentene+2-butene

2-hexene+propylene

1-pentene+1-pentene

4-octene+ethylene

1-pentene+propylene

2-hexene+ethylene

1-pentene+2-pentene

3-heptene+propylene

1-pentene+2-pentene

2-hexene+1-butene

In one embodiment of the present invention, the feed to the metathesisreaction zone would have a molar ratio for 1-butene:2-butene in therange of from about 1:6 to about 6:1. In another embodiment of thepresent invention, the molar ratio would be about 1:1 to about 3:1. Inanother embodiment of the present invention, the molar ratio would beabout 2:1. With such molar ratios the likelihood of the followingreactions are enhanced:

1-butene+2-butene

2-pentene+propylene

2-pentene+1-butene

3-hexene+propylene

1-butene+1-butene

3-hexene+ethylene

2-butene+ethylene

propylene+propylene

These equilibrium reactions favor the production of propylene and3-hexene. In one embodiment of the invention the concentration ofhexenes and higher olefins in the metathesis product is up to 30 mole %.In another embodiment of the invention the concentration of hexenes inthe metathesis product is about three to about twenty mole %. In anotherembodiment of the invention the propylene:hexenes molar ratio is in therange of from about 1:1 to about 3:1. In another embodiment of theinvention the propylene:hexenes molar ratio is about 2:1.

Depending on the relative amounts of the olefins in the feed and therecycle stream, an olefin isomerization reactor can balance the amountsof olefins in the feed to the metathesis reactor to a molar ratio for1-butene:2-butene in the range of from about 1:6 to about 6:1. Anyolefin isomerization catalyst could be used, including basic metaloxides (i.e. magnesium oxide, calcium oxide, barium oxide, etc.).

The metathesis reaction may be in liquid phase or gas phase. Themetathesis reaction may be carried out at a temperature in the range offrom about 25° C. to about 400° C. and a pressure in the range of fromabout 0 psig to about 500 psig. Using catalysts at low temperature couldsuppress the isomerization of the olefins and increase the selectivityto C₂-C₆ olefins because fewer C₇₊, compounds are formed.

Since metathesis catalysts are susceptible to poisoning by a number ofimpurities which may be present in the feed (e.g., dienes, acetyleniccompounds, polar molecules, oxygenates, sulfur compounds, and nitrogencompounds), the feed may be purified prior to its introduction to thereactor. Guard beds may be placed upstream of the catalyst bed in orderto ensure removal of trace component poisons (e.g. water, oxygenates,sulfur compounds, and nitrogen compounds). U.S. Pat. No. 7,214,841discloses that metathesis catalysts are tolerant of butadiene levels upto 10,000 ppm. In one embodiment of the present invention the level ofall such non-diene impurities, contaminants and poisons (acetyleniccompounds; polar molecules, such as water; oxygenates; sulfur compounds;nitrogen compounds and combinations thereof) in the feed is no more than100 ppm.

After the metathesis reaction, the products are separated into at leastthree streams: a recycle stream of pentenes, butenes, and possiblyethylene, a product stream of propylene and a stream of hexene andhigher olefins for aromatization. Separation can be by any known methodin the art, e.g., reactive distillation, preferential boiling of theethylene and propylene, or a selective membrane.

The hexenes and higher olefins are aromatized to benzene and otheraromatics. Any catalyst suitable for the conversion of linear hexenes orhexanes (if the hexenes are hydrogenated) could be used for thearomatization step. Examples include zeolite catalysts, such asPt/KL-zeolite, Pt/CsGeZSM-5, Pt/KGeZSM-5 and Pt/KZSM-5.

The contact between the hexenes and higher olefins and the catalyst maybe at a liquid hourly space velocity in the range between 0.1 and 100h⁻¹, at a temperature in the range between 200 and 600° C. and at apressure in the range between 1 and 315 psia.

The invention having been generally described, the following examplesare given as particular embodiments of the invention and to demonstratethe practice and advantages thereof. It is understood that the examplesare given by way of illustration and are not intended to limit thespecification or the claims to follow in any manner.

Metathesis Catalysts:

1. Re₂O₇ (10 wt % Re-metal) on γ-alumina synthesized based on methodsdescribed in U.S. Pat. No. 6,130,1812. 4 wt % CoO, 14 wt % MoO₃ on alumina commercially available from StremChemicals3. 6 wt % WO₃ on silica synthesized based on methods described in U.S.Pat. No. 4,609,769

EXAMPLES

All runs were performed by passing a mixture of 1-butene and 2-butenethrough a vertical tubular reactor (½ inch in diameter and 18 inch inlength) positioned in a temperature controlled heating mantle. In eachrun the reactor contained a 5 g bed of the designated catalyst. Thereactor pressure was held at 40 psig, and the feed flows were set togive a WHSV of 1 hr⁻¹.

The calculated product distributions were made with the r_(equil) moduleof Aspen Plus, version 7, software.

Example 1 Re Catalyst at 48° C., 25% 1-butene

The feed with a 1-butene/2-butene molar ratio of 1:3 was passedcontinuously through a catalyst bed of Re₂O₇ on alumina (10 wt %Re-metal) which was maintained at 48° C. The catalyst was previouslyactivated in air at 500° C. Reaction product analysis after 24 hourstime on stream revealed a total butenes conversion of 43% with anequilibrium distribution of products. The selectivity towards propylenewas found to be 49% while the selectivity towards hexenes was 4%. Theratio of cis-3-hexene to trans-3-hexene was found to be 1:6. Acomparison between the calculated equilibrium product distribution andthe observed values is shown in FIG. 2.

Example 2 Re Catalyst at 48° C., 75% 1-butene

The feed with a 1-butene/2-butene molar ratio of 3:1 was passedcontinuously through the catalyst bed of Re₂O₇ on alumina (10 wt %Re-metal) which was maintained at 48° C. The catalyst was previouslyactivated in air at 500° C. Reaction product analysis after 19 hourstime on stream revealed a total butenes conversion of 59% with anequilibrium distribution of products. The selectivity towards propylenewas found to be 46% while the selectivity towards hexenes was 20%.Again, the ratio of cis-3-hexene to trans-3-hexene was found to be 1:6.A comparison between the calculated equilibrium product distribution andthe observed values is shown in FIG. 3.

Example 3 Co/Mo Catalyst at 97° C., 50% 1-butene

The feed with a 1-butene/2-butene molar ratio of 1:1 was passedcontinuously through the catalyst bed of CoO/MoO₃ on alumina (3.5 wt %CoO, 14 wt % MoO₃) which was maintained at 97° C. The catalyst waspreviously activated in nitrogen at 500° C. Reaction product analysisafter 9 hours time on stream revealed a total butenes conversion of 57%with an equilibrium distribution of products. The selectivity towardspropylene was found to be 46% while the selectivity towards hexenes wasapproximately 10%. A comparison between the calculated equilibriumproduct distribution and the observed values is shown in FIG. 4.

Example 4 WO₃ Catalyst at 296° C., 50% 1-butene

The feed with a 1-butene/2-butene molar ratio of 3:1 was passedcontinuously through the catalyst bed of WO₃ on silica (6 wt % WO₃)which was maintained at 296° C. The catalyst was previously activated innitrogen at 600° C. Reaction product analysis after 45 minutes time onstream revealed a total butenes conversion of 42% with an equilibriumdistribution of products. The selectivity towards propylene was found tobe 48% while the selectivity towards hexenes was 5%. The experimentalproduct distribution is shown in FIG. 5.

Aromatization

Catalyst: The catalyst was Pt/CsGeZSM-5 made by the methods disclosed inU.S. Patent Application Publication no. 2008/0255398.

Examples

A mixture of 14% cis-3-hexene and 86% trans-3-hexene was blended withn-hexane at varying levels and used as reactor feed. This 1:6 ratio ofcis:trans is close to the calculated equilibrium ratio of 12% cis and88% trans at this temperature. Different levels of hexene were usedsince the product from the metathesis process may possibly be blendedwith other C₆ feeds. The testing conditions below were used. GC data forthe individual compounds show that much of the 3-hexene is quicklyhydrogenated to n-hexane, some of which is dehydrogenated again to 1-and 2-hexenes. The procedure was repeated for 1-hexene. The results areshown in Table 1 below.

TABLE 1 1% Pt/CsGeZSM-5 catalyst. Conversion and aromatics selectivitymeasured at T = 515° C., P = 1 atm, H₂/HC in feed = 0.75, LHSV = 8.6hr⁻¹. Feed composition (wt. %) C₆ conversion n-hexane 3-hexenes 1-hexenearomatics selectivity 100 0 0 18 91 90 10 0 17 91 70 30 0 20 89 0 100 030 86 90 0 10 23 89 70 0 30 25 89 0 0 100 30 89

From these data we can see that both 1-hexene and the 3-hexenes areconverted to benzene with good selectivity and the rate of conversion ofboth is somewhat higher than that of n-hexane. Selectivity to aromatics(which were predominantly benzene) declines slightly as the level ofolefin increases, probably because the olefin cracks somewhat fasterthan the paraffin. The rate of deactivation was unaffected by olefinconcentrations up to 30%.

FIG. 1 shows an embodiment of the present invention. A hydrocarbon feedcontaining butenes (1-butene, 2-butene and, optionally, isobutene) isintroduced into the process. The feed may also contain pentenes. Thefeed may be combined with a recycle stream containing product butenes.Depending on feed composition before combination with the recycle and oncomposition of the recycle stream, either the recycle stream or the feedafter combination with the recycle may be modified in an isomerizationreactor to obtain a desired molar ratio of 1-butene:2-butene for thefeed composition into the metathesis reactor, i.e., the isomerizationreactor may be located in the recycle stream or in the feed stream aftercombination with the recycle. Before being introduced into themetathesis reactor the feedstream may be combined with recycle streamscontaining pentenes and ethylene. The feed introduced into themetathesis reactor may contain butenes (1-, 2- and iso), pentenes andethylene. Some possible metathesis reactions are disclosed above. Asnoted above, any metathesis catalyst should be effective for the presentinvention. The desired products of the metathesis reaction are propyleneand hexene. Pentenes, product butenes and ethylene are byproducts whichmay be recycled (along with unreacted butenes). Ethylene may beseparated for further processing into other compound and productsinstead of being recycled. In addition to the products for recycle, thereaction products from the metathesis reactor are separated into atleast two product streams, one containing propylene and one containinghexenes and higher olefins, and recycle stream(s) of unreacted butenesand pentenes. As noted above, ethylene may be recycled or product. Anyseparation technique known in the art to yield these products andrecycle may be used in the present invention. After separation, thehexene product stream is introduced into an aromatization reactor forconversion to aromatics. The hexenes are converted to benzene. Thehigher olefins, such as heptenes and octenes, are converted to toluene,xylenes, ethylbenzene and styrene. Any aromatization catalyst known inthe art for aromatizing olefins may be used in the present invention.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

1. A process for producing propylene and aromatics comprising: a)reacting a hydrocarbon feed mixture comprising 1-butene, 2-butene and,optionally, isobutene in the presence of a metathesis catalyst atconditions to produce a product comprising ethylene, propylene, productbutenes, pentenes, hexenes and higher olefins; b1) separating from theproduct and recycling the pentenes, the butenes, and optionally, theethylene; b2) separating the propylene from the product; b3) separatingthe hexenes and higher olefins from the product; and c) reacting thehexenes and higher olefins in the presence of an aromatization catalyst,wherein in step a) the molar ratio of 1-butene:2-butene in thehydrocarbon feed is in the range of from about 1:6 to about 6:1.
 2. Theprocess of claim 1 wherein the metathesis catalyst is WO₃ on SiO₂, MoO₃on Al₂O₃ or Re₂O₇ on Al₂O₃.
 3. The process of claim 1 wherein thearomatization catalyst is Pt/KL-zeolite, Pt/CsGeZSM-5, Pt/KGeZSM-5 orPt/KZSM-5.
 4. The process of claim 1 wherein the aromatization catalystis Pt/CsGeZSM-5.
 5. The process of claim 1 wherein the concentration ofhexenes and higher olefins is up to 30%.
 6. The process of claim 1wherein the hydrocarbon feed mixture additionally comprises pentenes. 7.The process of claim 1 wherein the molar ratio of 1-butene:2-butene inthe hydrocarbon feed is about 2:1.
 8. The process of claim 1 wherein thehydrocarbon feed comprises a level of non-diene impurities, contaminantsand poisons of no more than 100 ppm.
 9. The process of claim 8 whereinthe impurities, contaminants and poisons are acetylenic compounds, polarmolecules, oxygenates, sulfur compounds, nitrogen compounds orcombinations thereof.
 10. The process of claim 9 wherein the polarmolecule is water.
 11. The process of claim 1 wherein metathesis iscarried out at a temperature in the range of from about 25° C. to about400° C. and a pressure in the range of from about 0 psig to about 500psig.
 12. The process of claim 1 wherein aromatization is carried out ata liquid hourly space velocity in the range between 0.1 and 100 h⁻¹, ata temperature in the range between 200 and 600° C. and at a pressure inthe range between 1 and 315 psia.
 13. The process of claim 1 wherein,after the butenes are recycled to the hydrocarbon feed, the hydrocarbonfeed is isomerized to a 1-butene:2-butene molar ratio in the range offrom about 1:6 to about 6:1 prior to being introduced into themetathesis reactor.
 14. The process of claim 1 wherein the butenes areisomerized prior to being recycled to the hydrocarbon feed and thehydrocarbon feed has a 1-butene:2-butene molar ratio in the range offrom about 1:6 to about 6:1 prior to being introduced into themetathesis reactor.
 15. The process of claim 1 wherein thepropylene:hexenes molar ratio in step a) is about 2:1.
 16. The processof claim 1 wherein the propylene:hexenes molar ratio in the metathesisproduct is in the range of from about 1:1 to about 3:1.
 17. The processof claim 1 wherein the concentration of hexenes in the metathesisproduct is about three to about twenty mole %.